The present invention particularly relates to an automotive radio receiving circuit with a search tuning mechanism suitable for use in a vehicle.
Heretofore, a radio receiving circuit with a search tuning mechanism has been well known, in which a motor is started to continuously vary variable tuning elements to tune them to the incoming broadcasting radiowave signal. The conventional search tuning operation has been carried out in the procedure such that when the arrival of an output level of an intermediate frequency signal from an intermediate frequency amplifier relative to a predetermined value is detected, a trigger pulse is provided by which pulse the search tuning operation is interrupted.
However, according to the above-mentioned search tuning system, a significant deviation in time of producing a trigger pulse occurs according to the magnitude of the electric field strength possessed by the incoming radiowave signal fed to the radio receiving circuit, and consequently, the search tuning operation is interrupted at a frequency deviated from the center frequency of the incoming radiowave signal and the condition becomes sometimes locked to the receiving state, which is very inconvenient.
That is, for example, in the case where the output level from the intermediate frequency amplifier which produces a trigger pulse is set to a low level in advance, when a receiving radiowave signal from a certain broadcasting station is subject to search tuning operation within an area where the electric field strength is extremely great, a trigger pulse is produced at a frequency greatly deviated from the center frequency of the receiving radiowave signal to select the station. Therefore, even if the same radiowave signal is received, the same signal cannot be received when the vehicle has moved to another area where the electric field strength is small, and the search tuning operation must be carried out again or erroneous tuning due to a field noise unexpectedly occurs during the search tuning operation, thus sometimes disadvantageously interrupting the operation. Conversely, in the case where the output level of the intermediate frequency which produces a trigger pulse is set to a high level in advance, there is an inconvenience that in the area where the electric field strength of the radiowave signal which one desires to receive, the desired station cannot be selected.